A helium-neon laser has a power output of 1mW of light of wavelength 632.8 nm.
Q. Determine the number of photons emitted by the laser each second.

Options

A. `3.18xx10^(15)`
B. `4.5xx10^(16)`
C. `1.2xx10^(15)`
D. `2.9xx10^(17)`

Explanation

To determine the number of photons emitted by a helium-neon laser each second, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Given Values:**
- Power output of the laser, \( P = 1 \text{ mW} = 1 \times 10^{-3} \text{ W} \)
- Wavelength of the light, \( \lambda = 632.8 \text{ nm} = 632.8 \times 10^{-9} \text{ m} \)

2. **Calculate the Energy of a Single Photon:**
The energy \( E \) of a single photon can be calculated using the formula:
\[ E = \frac{hc}{\lambda} \]
where:
- \( h = 6.626 \times 10^{-34} \text{ J s} \) (Planck's constant)
- \( c = 3 \times 10^{8} \text{ m/s} \) (speed of light)

Plugging in the values:
\[ E = \frac{(6.626 \times 10^{-34} \text{ J s})(3 \times 10^{8} \text{ m/s})}{632.8 \times 10^{-9} \text{ m}} \]

3. **Calculate the Energy of a Single Photon:**
Performing the calculation:
\[ E = \frac{(6.626 \times 10^{-34})(3 \times 10^{8})}{632.8 \times 10^{-9}} \approx 3.14 \times 10^{-19} \text{ J} \]

4. **Calculate the Number of Photons Emitted per Second:**
The number of photons \( n \) emitted per second can be calculated using the formula:
\[ n = \frac{P}{E} \]
Substituting the known values:
\[ n = \frac{1 \times 10^{-3} \text{ W}}{3.14 \times 10^{-19} \text{ J}} \]

5. **Perform the Calculation:**
\[ n \approx 3.18 \times 10^{15} \text{ photons/second} \]

### Final Answer:
The number of photons emitted by the helium-neon laser each second is approximately \( 3.18 \times 10^{15} \) photons.

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